Function: fill-find-break-point

Move point to a proper line breaking position of the current line.
Don't move back past the buffer position LIMIT.

This function is called when we are going to break the current line
after or before a non-ASCII character. If the charset of the
character has the property `fill-find-break-point-function', this
function calls the property value as a function with one arg LIMIT.
If the charset has no such property, do nothing.